The present invention relates to rotary electric motors, more particularly to motors having first and second annular ring members concentrically arranged about an axis of rotation and separated from each other by an axial air gap, both of the members comprising groups of magnetically isolated structures, the groups of one of the members having permanent magnets affixed thereto and the other of the members comprising wound electromagnet poles.
Direct current motors have versatility in a wide range of applications. The availability of a battery power source for dc motor equipped devices facilitates a portability aspect that is not readily available for a-c motor drives. Electronic controls, such as microcontroller and microprocessor based systems, for a wide variety of functional applications have become commonplace. As development of the battery has progressed, and the versatility of electronic controls has expanded, the challenge of providing efficient direct current motor drives for vehicles, as a viable alternative to combustion engines, has become more compelling. U.S. Pat. No. 5,164,623 to Shkondin is one example of a proposed implementation in which a motor is mounted on the wheel of a vehicle for directly driving the vehicle. The patent proposes that such an arrangement can be applicable to electric cars, bicycles, wheelchairs and the like.
Electronically controlled energization of windings of direct current motors offers the prospect of more flexible management of motor characteristics. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption. U.S. Pat. No. 4,754,207 to Heidelberg et al. describes a direct current motor having a rotor composed of a continuous ring of a plurality of permanent magnets successively alternating in polarity. The stator, comprising a plurality of electronically switchable electromagnet poles, is circumferentially separated from the rotor magnets by a radial air gap. Several adjacent stator electromagnets form a phase group. The inward base portions of adjacent electromagnet poles in each group are in surface area contact with each other to form a continuous magnetic flux path. The electromagnetic circuit is broken at transition points between adjacent groups of electromagnets. Sensors detect relative rotational position between rotor and stator elements to control electronic switching of the individual electromagnet windings. Electromagnets belonging to a common group are switched simultaneously with one common electronic switching means per group. Windings of the electromagnets in adjacent groups are of different phases and are switched at different times.
Of concern in implementation of stator winding switched energization is the avoidance of unfavorable consequences such as rotation irregularities. For example, simultaneous switching of all motor phase windings can cause pulsating output torque. Alleviation of these effects, with varying success, can be obtained by appropriately switching all phases at different times or by simultaneously switching certain winding combinations that are distributed symmetrically about the stator periphery and bear certain positional relationships with the permanent magnet poles of the rotor. However, switching of adjacent windings at different times leads to detrimental effects if the windings are linked to a continuous magnetic circuit path, as the flux changes due to the changed energization of the winding of one pole effects the flux of an adjacent pole.
Heidelberg et al. alleviates this problem to some extent by grouping pluralities of stator poles in separate magnetic circuit paths. The magnetic circuit path discontinuity between adjacent groups effects an isolation of magnetic flux, thus reducing transformer like magnetic flux interference between groups. However, where all poles of a group are wound and switched simultaneously, a torque ripple effect can still exist. Heidelberg et al. provides modifications in which some poles of a group are not wound and/or the pole structure of all poles within a group are not of uniform configuration, thus deterring the effects of torque ripple and flux interference between adjacent poles. Such modifications sacrifice torque characteristics and power capability. If fewer poles are wound, flux generation capability is reduced. The unwound poles do not contribute to torque and can detrimentally interact with rotor permanent magnets. Non-uniform pole configuration modifications in Heidelberg et al. are coupled with non-uniform pole windings. Such configurations complicate the manufacturing process and compromise motor efficiency.
The above-identified copending related U.S. patent application Ser. No. 09/826,422 identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient flexible operating characteristics. In the particular vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience. The copending related U.S. application incorporates electromagnet poles as isolated magnetic structures configured in a thin annular ring to provide advantageous effects. With this arrangement, flux can be concentrated, with virtually no loss or deleterious transformer interference effects in the electromagnet cores, as compared with prior art embodiments. While improvements in torque characteristics and efficiency are attainable with the structure of the identified copending application, further improvements remain as an objective.
The present invention fulfills the above-described needs and provides further advantages. A rotary electric motor comprises rotor and stator members each configured as annular rings and concentric with respect to each other about an axis of rotation. Either of the rotor or stator members is formed of groups of electromagnet pole pairs, the groups substantially equidistantly distributed along the angular extent of the annular ring, each of the groups comprising magnetic material magnetically isolated and separated from the other groups. The other member comprises a plurality of groups of permanent magnet poles substantially equidistantly distributed with alternating magnetic polarity along the angular extent of the radial air gap formed between the members. The groups of permanent magnet poles each comprise a common magnetic return path that is separate and magnetically isolated from adjacent permanent magnet pole groups. The poles of each group of electromagnet pole pairs are wound, the windings together being switchably energized for driving electromotive interaction between the stator and rotor. Thus, an even number of poles, two per pole pair, are provided for each electromagnet group. The poles of each pole pair are oppositely wound to provide opposite north/south polarities.
As described in the related copending application, isolation of the electromagnet groups permits individual concentration of flux in the magnetic cores of the groups, with virtually no flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched. The lack of additional poles within the group eliminates precludes any such effects within a group.
By appropriately timing the switched winding energization for each of the pole pair groups, development of smooth electromotive force throughout the motor is attained. A precise optimum phase and sequence for timed switching of particular pole pair groups is dependent upon the particular structural configuration of electromagnetic poles, permanent magnet poles, spacing among various poles and other structural interrelationships. Upon determination of the optimum timed switching sequence for a specific motor configuration, implementation of a switching scheme can be made dependent upon relative position between rotor and stator. Switching may be performed by a mechanical commutator or electronic activation in response to signals generated by a position sensor. A wide variety of suitable sensors are well known in the art including, merely by way of example, reed switch sensors, capacitive sensors, hall effect sensors, optical sensors, and pulse wire sensors. Microprocessor controlled electronic switching affords precisely adjustable speed in a light weight structure. While various position sensing means are well known in the art, any of which may be employed to generate such signals, the use of a resolver has been found to be preferable. The resolver output can then be used by an encoder to encode signals for application to a microcontroller or microprocessor based control circuit.
The embodiments of the present invention provide yet additional advantages. The propulsion system comprises as main structural constituents, an electromagnet subsystem, permanent magnet clusters and enclosing back iron ring sections for the permanent magnet clusters. The permanent magnets and the ring portions form the rotor part of the motor, the permanent magnets being positioned inside the back iron ring sections.
Parameters of interest in the rotor are the grade of the magnet, the energy density and the overall magnetic characteristics of the magnet grade, the size and the dimensions of the magnet that can adjust the permanence and the overall operating condition of the magnet when it is part of the rotor, the temperature stability of the magnet, the finishing, coating and post processing steps taken in manufacturing of the magnets for the intended application, the stability of the magnetisation over the curvilinear surface of the magnet, uniformity of the radial polarisation of the magnet, the adjacent gap between two separate magnets, the mechanical features of the edges of the magnets, and the return flux path of the magnet as provided by the back iron ring section. The back iron ring sections are predominately a soft magnetic medium. They can be manufactured by various techniques from cast, compacted, sintered or powdered materials as well as ferromagnetic soft magnetic laminated silicon steels. For optimal operation, the back iron should have a high permeability and saturation flux density level preferably around 2.5 T.
In a configuration in which permanent magnets are fixed to a continuous back iron ring, such as in the above described copending application Ser. No. 09/826,422, with no excitation applied to any of the electromagnet phases, an equilibrium exists. The continuous iron ring experiences full magnetic flux saturation at the regions behind the regions where there are adjacent gaps between two magnets. If the ring is examined carefully, this flux saturation pattern is repeated within the bulk of the ring. The saturation flux density can be within 2.0 to 2.3 T. The dimension and the material grade of the iron ring can be modified in order to reduce saturation intensity. Under no electromagnet excitation the flux distribution pattern in the back iron ring is stable (not modulated), although subtle nominal variations could exist as there may be some negligible variations in the energy density values of each magnet positioned in the subassembly. However, during the excitation cycles of a given phase of the motor, a magnetic potential difference tends to build up between the poles of the electromagnet and the corresponding coupling permanent magnet. This potential differential tends to alter the flux pattern in the corresponding segment return path of the magnet. As this effect is localized, only the corresponding segment of the back iron path would experience a subtle reduction in the intensity of flux saturation. Since the excitation current is modulated under a specific PWM scheme, the reduction in the intensity of the saturation would undergo the same modulation pattern as the excitation. However, this variation is frequency dependent and is proportional to several key factors: 1. the frequency of the switching of each phase 2. the fundamental frequency of the PWM scheme and 3. the changing electrical duty cycle per phase. All of these effects contribute to the development and the propagation and modulation of strong eddy currents in the iron path and hence a skin effect within the bulk of the iron ring. These eddy currents tend to propagate around the path of the back iron and hence upset the equilibrium of other segments and hence cause unwanted lags in the excitation currents of other phases. The most dominant effect is the eddy current losses which are caused in the bulk of the iron path. The intensity of this eddy current loss can be mitigated by increasing the inherent electrical resistivity (or reducing the electrical conductivity of the material), or by changing the grade and the geometry or the placement of the back iron path, or by inducing barriers to the flow of the eddy currents. If the losses are predominant, one method of reducing the eddy currents would be to use laminated steel construction. However, as the iron ring is shared by all of the magnets, this alone my not entirely address the problem.
A solution of the present invention is to divide each back iron into segments (proportional to the pitch of a magnet cluster) such that there is a physical air gap between the back iron segments of two adjacent magnet clusters. Although this may reduce the effective flux link between the two adjacent magnets of different clusters (and in turn in the whole back iron ring), it isolates the effects of the eddy current along its propagation path. The orientation of these gaps in the back iron are configured to achieve the desired operation of the design.
With the additional isolation of permanent magnet groups, flux fields of both stator and rotor components are thus concentrated and focussed at the air gap for optimum electromotive interaction. Such interaction is particularly effective when the number of poles in each of the electromagnet and permanent magnet groups is the same. The maximum number of groups can be realized by employing two poles in each group. Interaction between single isolated pole pairs of the electromagnet member and permanent magnet member for all groups along the air gap contributes to high torque capability with efficient operation. Such efficiency coupled with light weight electronically switched winding energization significantly extends battery life.
While the present invention has useful applicability in various motor drive applications, it is advantageously suitable for a vehicle drive in which the rotor is structured to surround the stator, the rotor being secured to a housing for direct attachment to a vehicle wheel. The annular rotor is thus at a substantial radial distance from the axis of rotation. The rotor housing is journalled for rotation about a stationary shaft at the axis of rotation through bearings. In an embodiment in which the rotor comprises permanent magnets, a large number of groups of permanent magnets can be substantially evenly distributed along the annular ring, affixed to a ring of nonmagnetic material such as aluminum. As a programmed microprocessor has the capability of generating extremely high rate of switching signals, a wide vehicle speed and torque range is available without need for transmission gear shifting. The groups of separate electromagnets form a relatively thin annular stator ring that is spaced from the axis of rotation by a radial distance, which preferably is substantially greater than the radial dimension between inner and outer diameter boundaries of the stator ring. The separated groups of electromagnets are removably secured to plate members that are also affixed to the stationary shaft. The configuration of the present invention, wherein electromagnet poles form isolated magnetic structures formed in a thin annular ring, has been found to provide further advantageous effects. With this arrangement, flux can be concentrated, with virtually no loss or deleterious transformer interference effects, at the radial air gap for optimum interaction with the permanent magnet rotor. As a result, extremely high torque is available with a high efficiency that significantly extends battery life.
The stator structure of the present invention provides an additional advantage in that access to and replacement of an individual stator group is facilitated. Thus, if a particular stator winding group should become damaged, for example by a fault in the pole structure or winding, the individual stator group can be replaced without removing or replacing the entire stator unit. As a related advantage, it has been found that use of a large number of single pole pair stator groups and rotor groups permits the motor to continue to operate in a satisfactory manner even if a particular electromagnet pole group energization fails.
A further advantage of the present invention is that, to a large extent, stator and rotor pole face dimensions and spacings between poles are relatively independent of each other. A timed switched energization scheme can be programmed to be optimized for a particular structural configuration. In the preferred vehicle drive embodiment, described above, an odd number of stator groups is utilized. The stator poles have pole faces at the air gap that are of substantially uniform angular extent. The permanent magnet rotor pole faces are of substantially equal angular dimensional extent at the air gap, which is different from the stator pole face dimension. The angular distance between the centers of the pole faces within each stator group is substantially uniform throughout the periphery of the stator and differs from the angular distance between the centers of the stator pole faces of adjacent groups. The angular distance between the centers of the pole faces of each stator group also is different from the angular distance between the centers of adjacent permanent magnet poles of the rotor. Gaps between adjacent stator pole faces within each group are substantially equal for all groups and different from gaps between adjacent stator groups. The rotor pole faces are separated substantially uniformly by gaps, both within and between groups, the gaps between adjacent rotor pole faces preferably being different from the gaps between adjacent stator pole face within a stator group.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.